The present invention relates to a broadcast signal receiver, and more particularly, to a broadcast signal receiver where a low-noise amplifier is installed in front of a tuner so as to improve the reception sensitivity of the receiver.
In a conventional broadcast signal receiver, the noise figure of the system as a whole largely depends on its tuner. As the system's noise figure becomes smaller, the receiver's sensitivity is enhanced. Therefore, a tuner having a low noise figure is highly desirable.
To lower the tuner noise figure, high-quality tuner components are required, which increases manufacturing costs. On the other hand, due to the innate structure of the tuner, the insertion loss of a filter installed at the front end of the tuner limits the obtainable degree of noise figure improvement.
FIG. 1 shows a conventional broadcast signal receiving device.
Referring to FIG. 1, reference numeral 10 denotes a tuner for receiving a radio frequency (RF) signal from an antenna and for tuning a desired channel to output an intermediate frequency (IF) signal, reference numeral 12 denotes a surface acoustic wave (SAW) filter for limiting the IF bandwidth signal, reference numeral 14 denotes an IF signal processor for outputting the bandwidth-limited IF signal as a video signal.
The system noise figure NF.sub.sys of the conventional broadcast signal receiver shown in FIG. 1 can be expressed as follows. ##EQU1## Here, NF.sub.tun is the tuner's noise figure, NF.sub.fil is the SAW filter's noise figure, NF.sub.amp is the total noise figure of the IF stages (IF signal processor), G.sub.tun is tuner gain, and G.sub.fil is the loss (negative gain) of the SAW filter.
As can be seen in expression (1), the noise figure of an overall system is effectively determined by the noise figure and gain of the first stage of the system. That is, the noise figure (NF.sub.sys) of a conventional overall system is determined by the tuner's noise figure (NF.sub.tun) and gain (G.sub.tun),
Here, the tuner, which plays the most significant role in determining the noise figure of an overall system, has a problem in that both the noise figure and the gain vary according to the channel of a broadcast signal which is received, so that the sensitivity may worsen according to the specific channel received.
In addition, the internal constitution of a conventional tuner for tuning an NTSC broadcast signal consists of respective filters for the VHF and UHF bands at the front end of the tuner, as is widely known. Therefore, the noise figure of the system is inevitably increased due to the insertion loss (negative gain) of the filters.
In the meantime, for relatively weak input signals, a simple booster stage is employed in front of the tuner to compensate for the low input level. However, the noise figure of the booster stage (8-12 dB) is generally larger than that of the tuner (4-10 dB), so that receiver sensitivity suffers. An RF amplifier having a noise figure lower than that of the booster stage can be installed in front of a tuner, without employing a booster stage, but most such amplifiers cannot cover the entire receiving band, say, 50-900 MHz, of a typical broadcast signal receiver. Thus, one RF amplifier for VHF channels and another for UHF channels are required, which complicates the circuitry.